for MODX REVO traverse up the amount of levels you want from the current resource and get its id. @example: [[GrandParent? &levels=`2`]]

GrandParent (MODX REVO SNIPPET)

<?php
/**
 * [[GrandParent]]
 * 
 * traverse up the amount of levels you want from the current resource and get its id.
 * @example: 
 * [[GrandParent? &levels=`2`]]
 * 
 * inspired by http://www.sepiariver.ca/blog/modx-web/modx-quick-tip-get-id-of-grandparent
 */

$id = $modx->resource->get('id'); 

$parents = $modx->getParentIds($id, $levels); 
$grandParent = $parents[$levels-1];

return $grandParent;